1. Field of the Invention
The present invention relates to a printer used in conjunction with a point-of-sale (POS) terminal where the printer can print to roll paper for issuing receipts, for example. The present invention relates more specifically to a roll paper loading mechanism whereby replacing and loading roll paper rolls in such a printer can be accomplished easily and appropriately.
2. Description of the Related Art
Printers for printing to roll paper are commonly used with POS terminals, and the ability to easily load and replace roll paper in such printers is particularly desirable. Roll paper loading mechanisms for accomplishing this task with the cover of the printer's roll paper holding unit fully open have been proposed. Japan Examined Patent Publication (kokoku) 6-79855, for example, teaches a recording apparatus comprising this type of roll paper loading mechanism.
In the recording apparatus taught in the cited Publication, a platen roller is pressed against a thermal head, and both ends of the platen roller support shaft are held in oblong holes formed near the edge of side support members. These support members are pivotable in conjunction with a cover, which functions as the cover of the roll paper holding unit. Fully opening this cover enables the roll paper to be easily mounted in the roll paper holding unit. Opening the cover also separates the platen roller from the thermal head, thereby enabling the roll paper leader to be easily positioned between the platen roller and thermal head.
When the cover is then closed, engaging members formed on the edges of the cover press both ends of the platen roller support shaft towards the thermal head. Because the ends of the support shaft are supported in oblong holes, this pressure on the shaft ends causes the platen roller to move towards and against the thermal head with the pressure therebetween maintained by a spring supporting the thermal head. As a result, closing the cover automatically results in the roll paper being clamped between the thermal head and platen roller.
Problems such as described below, however, remain to be solved with the roll paper loading mechanism described above.
First, the movement of the platen roller when the cover is closed is determined by the length of the oblong holes in which the platen roller support shaft is supported, and the range of platen roller movement is generally limited when the platen roller is supported in such holes.
When the cover is closed, the edges of the platen roller supports should therefore be as close to the thermal head as possible, and the supports must therefore be made longer than otherwise necessary. Increasing the length of the supports, however, increases the radius of rotation. The top of the roll paper holding unit opening must therefore be opened wide enough to prevent interference with rotation of the platen roller support. This limits the area above this top opening that can be occupied for some other purpose, and thus greatly limits the available design and layout options.
Second, the ends of the platen roller support shaft must be pressed by the engaging members of the cover to press the platen roller against the thermal head. Depending on how each of these engaging members actually engages the shaft ends, different friction loads can be applied to opposite shaft ends, resulting in interference with smooth platen roller rotation.
Third, there are cases in which the platen roller cannot be accurately positioned and pressed against the thermal head by its movement along oblong holes in the support members. For example, if the manufacturing precision of the engaging members is poor, platen roller movement will be different at each end. As a result, the spring-mounted thermal head may not press against the platen roller with appropriate force at all points along the roller, and in extreme cases the thermal head may only contact the platen roller on one side of the thermal head.
This platen roller positioning error does not normally pose a particularly great problem in printers equipped with a spring-mounted thermal head. In printers that use an inkjet head or wire dot impact head, however, the print head to platen roller gap is more critical. More specifically, it is the platen roller or other platen member, including paper guide members functioning as a platen, that determines the distance of the paper from the print head, i.e., the distance from the print head to the surface that is printed on, in this case. It is therefore necessary to position the platen roller or other platen member with a uniform gap to the print head so that the printing surface of the paper is passes the print head with an appropriate gap between the paper surface and the print head. If the position of the platen roller or paper guide varies in a printer having a print head requiring a particular platen gap, problems that cannot be ignored, including print quality deterioration, can occur.
It is, therefore, an object of the present invention is to provide a roll paper loading mechanism for a printer, which is capable of solving the above mentioned problems in the conventional printer.